Physical Structure

In short: The lowest (1st) layer of the OSI model — the actual physical transmission of bits as electrical signals, light pulses or radio waves.

In more detail: This is about cable types (Cat cables, fibre, coax), connector types, voltage levels and radio frequencies — a pure hardware level without any understanding of addresses or content. A broken cable or a disturbed radio connection is a classic layer 1 problem.

In Depth

The physical layer knows no concepts such as addresses, packets or senders — it exclusively transmits a raw sequence of bits (ones and zeros) as a physical signal. With copper cables this happens via voltage differences, with fibre (fibre) via light pulses of different intensity/wavelength, with radio (Wi-Fi, Bluetooth) via modulated electromagnetic waves. Exactly how a bit is translated into a signal (encoding), connector types (e.g. RJ45), maximum cable lengths and supported transmission speeds are defined here — with Cat cables, for example, the supported frequency and therefore bandwidth rises with each category (Cat5e, Cat6, Cat6a, …).

Because this layer works purely physically, its typical failure patterns are also mechanical/electrical: broken or overly long cables, defective connectors, electromagnetic interference (e.g. from power cables laid in parallel), or with radio simply too great a distance/too many obstacles between sender and recipient. In troubleshooting, a network engineer therefore typically checks this lowest level first (“is the cable even plugged in properly?”) before looking for more complex causes at higher layers.

Signal transmission and limits

Every physical medium has a maximum cable length beyond which the signal is attenuated so strongly that it can no longer be detected reliably — with copper Ethernet this limit is around 100 metres, which is why repeaters or switches are used to refresh the signal over longer distances. Fibre (fibre) overcomes this limitation, because light is attenuated much less than electric current — distances of several kilometres without signal refreshing are possible, which is why fibre is used above all for backbone connections between buildings or data centres, while copper cables (Cat cables) still mostly dominate for connecting end devices.

Bit rate versus baud rate

A common misconception: the pure signal frequency (baud rate, how often the signal state changes per second) doesn’t necessarily correspond to the actually transmitted bandwidth in bits per second — modern encoding methods transmit several bits per signal change, for example by distinguishing several voltage levels instead of just two. That’s why the theoretical maximum speed of a medium can’t simply be derived from the frequency alone, but depends on the specific encoding method and the number of wire pairs used (with copper cables).

See also: OSI model, Cat categories and specifications